Method of production of a methionine salt

ABSTRACT

A reaction system suitable for production of a methionine salt contains a reactive rectification column containing a weir having a height of 100 mm or more.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of production of a methionine salt, inparticular the production of a methionine salt starting from theprecursors 3-methylmercaptopropionaldehyde (MMP) and hydrogen cyanide(HCN) or starting from at least one component that can be prepared fromthese raw materials, such as methylmercaptopropionaldehyde-cyanohydrin(MMP-CN). In particular the invention relates to the alkaline hydrolysisof 5-(2-methylmercapto)-hydantoin in a column. The invention furtherrelates to a reaction system suitable for this method, comprising areactive-rectification column, and the use of the reaction system.

2. Discussion of the Background

The industrial synthesis of racemic methionine (mixture of 50%L-methionine and 50% D-methionine) starts from petrochemical rawmaterials, in particular propene, sulphur, methane and ammonia.According to usual methods, the precursor3-methylmercaptopropionaldehyde is thus prepared via the precursorsacrolein, methylmercaptan and hydrogen cyanide. Then this aldehyde isconverted with ammonia, carbon dioxide and hydrogen cyanide to5-(2-methylmercapto)-hydantoin, alkaline hydrolysis of which leads to analkali salt of methionine. Neutralization with an acid such as carbondioxide or sulphuric acid gives racemic methionine, several hundredthousand tonnes of which are produced annually.

A conventional method is based on the use of circulated alkalinepotassium salts for hydrolysis of 5-(2-methylmercapto)-hydantoin.Unwanted by-products lead to neutral potassium salts, which are then nolonger available to the alkaline hydrolysis reaction. These must beremoved from the potassium-containing circulating solution. Theassociated potassium losses must be compensated by using KOH. Anotherside reaction leads to the formation of4-methylmercapto-2-hydroxybutanoic acid and therefore to a loss ofyield. 4-Methylmercapto-2-hydroxybutanoic acid is also present asneutral potassium salt in the bottoms product of reactive distillationand therefore disturbs the circulation of alkaline potassium. Thisby-product is therefore unsuitable for supporting the hydrolysis of5-(2-methylmercapto)-hydantoin and must be removed from the potassiumcircuit, which is associated with further losses of raw materials.

It is known from the background art that methionylmethionine (alsocalled methionine dipeptide) is a by-product in methionine production byhydrolysis of hydantoin (e.g. EP 2 133 329 A2, EP 0839 804 B1) and formsin the following way (formula I).

A two-stage hydantoin hydrolysis for reducing the formation ofmethionine dipeptide is described in EP 2 133 329 A2. In this, the firststage takes place in a flow tube with gas outlet and the second stage ina stirred reactor, with total residence times of 20-60 minutes. Thisgives a product distribution of 91 mol. % methionine and 9 mol. %methionine dipeptide. The high proportion of methionine dipeptide isvery unfavourable and requires further process steps to reduce losses ofyield. Thus, EP 2 186 797 A1 describes expensive heat treatment at150-200° C. of the concentrates of the mother liquors from methioninecrystallization, to cleave methionine dipeptide hydrolytically back tomethionine. For this, after crystallization, which takes place at 20°C., the mother liquor must be heated again to the required hightemperatures, which is energetically unfavourable.

JP 2006-206534 A describes the depletion of NH₃ from a process solutionproduced in an additional process step by hydrolysis of hydantoin atnormal pressure and therefore at about 100° C. by means of a platecolumn. This is a normal distillation of NH₃, having no effect on thehydrolysis reaction. A drawback of this procedure is that the NH₃ thusremoved is no longer available for hydantoin synthesis. Moreover, thesolution thus treated is heavily diluted, as stripping steam at apressure of 5 bar(excess) (corresponding to 158° C.) is used, which isdisadvantageous for further processing.

EP-A-1710232 and EP-A-1256571 describe a method of production ofD,L-methionine from 5-(2-methylmercapto)-hydantoin, wherein the processstages 5-(2-methylmercaptoethyl)-hydantoin formation and methioninateformation (alkaline hydrolysis) can be operated continuously andintegrated successively in a fully continuous process. How a column forhydrolysis of Met-hydantoin should be designed technically to minimizeby-product formation has not yet been described in the background art.Furthermore, there is no description of how the NH₃ circuit betweenhydantoin formation and hydantoin hydrolysis is to be operated loss-freeat minimized energy expenditure. The hydrolysis process can be carriedout in a steam-heated column, wherein the5-(2-methylmercaptoethyl)-hydantoin solution is advantageously fedcontinuously to the top of the column at a rate such that the hydrolysisproduct, potassium methioninate solution, can accordingly be withdrawnat the bottom of the column after quantitative hydrolysis. The motherliquor can be reused after separating the methionine solids. The gaseousconstituents (steam, ammonia and carbon dioxide) can be discharged atthe top of the column and can be used for restoration of the aqueousammonia/carbon dioxide solution for production of5-(2-methylmercaptoethyl)-hydantoin. It is described as being especiallyadvantageous, for avoiding by-products, to carry out the hydrolysisright at the start in the presence of alkali and carbon dioxide, i.e. inparticular there is a mixture of alkali compounds, in particular alkalihydrogen carbonate, alkali carbonate, alkali hydroxide. To achievecomplete conversion of the valuable starting material MMP to hydantoin,a molar ratio of 1.005-1.02 mol/mol HCN/MMP is employed in the methoddescribed. As a result of hydrolysis of unreacted hydrogen cyanide, inthe column there is production of NH₃ by the following reaction (formulaII):

HCN+2H₂O→NH₃+HCOOH   II

The resultant formic acid is located as non-volatile potassium formateat the bottom of the reaction column; it thus displaces the requiredalkaline carbonates and therefore disturbs the alkaline potassiumcircuit.

SUMMARY OF THE INVENTION

The problem to be solved by the invention was therefore to provide animproved method, relative to the background art, for producing amethionine salt starting from the raw materialsmethylmercaptopropionaldehyde and hydrogen cyanide or starting from atleast one compound that can be prepared from these raw materials, suchas methylmercaptopropionaldehyde-cyanohydrin.

The problem is solved by the reaction system according to the invention,use thereof according to the invention and the method according to theinvention.

In one embodiment, the present invention relates to a reaction system,comprising:

a reactive rectification column comprising

-   -   a weir having a height of 100 mm or more;

wherein said reaction system is suitable for production of a methioninesalt.

In another embodiment, the present invention relates to a method forcontinuous production of a methionine salt, comprising:

reacting 3-methylmercaptopropionaldehyde and hydrogen cyanide or acomponent that can be produced therefrom, thereby obtaining a solutioncontaining 5-(2-methylmercaptoethyl)-hydantoin;

alkaline hydrolysing the 5-(2-methylmercaptoethyl)-hydantoin to amethionine salt in a reactive rectification column, wherein only thesolution containing 5-(2-methylmercaptoethyl)-hydantoin is fed on atopmost plate of the reactive rectification column and an alkalinecirculating solution is fed on a plate located under the topmost plate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1, top, shows a schematic top view of a sieve plate and, bottom, inone section, the side view of a column of a preferred embodiment.

FIG. 2 shows a flow chart of a plant including a sieve plate columnaccording to an embodiment of the invention.

FIG. 3 shows the composition on the various sieve plates in a column ofone embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The ranges stated below include all values and subvalues between theupper and lower limit of the ranges.

The reaction system has the following features:

-   -   1. Reaction system for production of a methionine salt,        comprising a reactive-rectification column with a weir height of        100 mm or more    -   2. Reaction system according to 1, wherein the weir height is in        the range from 100 to 1000 mm, preferably in the range from 150        to 700 mm,        -   the plate spacing is in the range from 500 to 1000 mm,        -   the ratio of column diameter to weir length is in the range            from 1.1 to 1.3,        -   the ratio of the cross-sectional area to the area through            which gas flows is in the range from 1.5 to 2 and        -   the number of plates is in the range from 15 to 25,            preferably in the range from 18 to 20.    -   3. Reaction system according to 1 or 2, wherein the        reactive-rectification column is a sieve plate column,        perforated plate column, valve plate column or bubble plate        column.    -   4. Reaction system according to 3, wherein, in the        reactive-rectification column the ratio total area of all        holes/area through which gas flows is in the range from 0.04 to        0.08 and        -   the diameter of the individual holes in the sieve plate is            in the range from 5 to 10 mm.    -   5. Reaction system according to one of the above numbers,        wherein the weir heights ensure an average residence time of the        respective mixture of less than 0.5 min per plate.    -   6. Reaction system, wherein the reaction system further        comprises at least one reactive absorber and optionally a second        reactor for the production of        5-(2-methylmercaptoethyl)-hydantoin.    -   7. Reaction system according to 5, wherein the reactive absorber        is a jet washer system.    -   8. Reaction system according to one of the numbers 1 to 7,        wherein zirconium is used in the reactive-rectification column        and/or in the reactive absorber and/or in the second reactor as        material for parts in contact with the product.

The method has the following features:

-   -   1. Method for continuous production of a methionine salt,        wherein the following steps are carried out:        -   reaction of 3-methylmercaptopropionaldehyde and hydrogen            cyanide or of a component that can be produced therefrom,            wherein a solution containing            5-(2-methylmercaptoethyl)-hydantoin is obtained;        -   alkaline hydrolysis of the            5-(2-methylmercaptoethyl)-hydantoin obtained to a methionine            salt in a reactive-rectification column, wherein only the            solution containing 5-(2-methylmercaptoethyl)-hydantoin is            fed on the topmost plate of the reactive-rectification            column and an alkaline circulating solution is fed on a            plate under that, preferably on the 2nd plate from the top.    -   2. Method according to 1, wherein the alkaline circulating        solution contains an alkali carbonate, preferably potassium        carbonate.    -   3. Method according to 1 or 2, wherein water, ammonia and CO₂        are removed from the top of the reactive-rectification column        and the NH₃ removed is condensed completely or partially for use        in the synthesis of 5-(2-methylmercaptoethyl-hydantoin.    -   4. Method according to one of the numbers 1 to 3, wherein the        concentration of ammonia in the bottom of the rectification        column is less than 120 ppm, preferably less than 100 ppm and        most preferably less than 80 ppm.    -   5. Method according to one of the numbers 1 to 4, wherein the        conversion to 5-(2-methylmercaptoethyl)-hydantoin is carried out        in a reactive absorber and then in a second reactor, preferably        in a second reactor designed as a flow tube.    -   6. Method according to one of the numbers 1 to 5, wherein the        temperature of the reaction mixture at the outlet of the        reactive-rectification column is in the range from 180° C. to        190° C.    -   7. Method according to one of the numbers 1 to 6, wherein the        temperature of the gas phase at the top of the        reactive-rectification column is in the range from 160° C. to        170° C.    -   8. Method according to one of the above numbers 1 to 7, wherein        alkaline hydrolysis is carried out at a pressure in the range        from 8 bar(excess) to 10 bar(excess).    -   9. Method according to one of the numbers 1 to 8, wherein steam        is used as heating and stripping medium in the        reactive-rectification column.    -   10. Method according to one of the numbers 1 to 9, wherein the        method is carried out in a reaction system according to one of        the numbers 1 to 8.

The invention also relates to the use of the reaction system for theproduction of methionine.

Reference is made expressly to the preferred embodiments of theinvention presented in the specification.

The invention relates to a method for continuous production of amethionine salt, wherein 3-methylmercaptopropionaldehyde and hydrogencyanide (HCN) or a component that can be produced therefrom are reacted,wherein a solution containing 5-(2-methylmercaptoethyl)-hydantoin isobtained and wherein an alkaline hydrolysis of the5-(2-methylmercaptoethyl)-hydantoin obtained to a methionine salt iscarried out in a reactive-rectification column, wherein only thesolution containing 5-(2-methylmercaptoethyl)-hydantoin is fed on thetopmost plate of the reactive-rectification column and an alkalinecirculating solution is fed on a plate located under that, preferably onthe 2nd plate from the top. The alkaline circulating solution preferablycontains an alkali carbonate, preferably potassium carbonate.

The reaction of the components 3-methylmercaptopropionaldehyde, hydrogencyanide and ammonia and carbon dioxide, or of components from which theaforementioned components can be produced, to5-(2-methylmercaptoethyl)-hydantoin takes place optionally in thepresence of water. In order to achieve conversion ofmethylmercaptopropionaldehyde that is as complete as possible, thestarting substances hydrogen cyanide and methylmercaptopropionaldehydeare preferably used in the molar ratio of 1.005-1.02 mol/mol HCN/MMP.

Preferred starting substances for the production of5-(2-methylmercaptoethyl)-hydantoin (also called hydantoin derivative orhydantoin for short) are 3-methylmercaptopropionaldehyde, hydrogencyanide, ammonia and carbon dioxide. By-products of this reaction arethe components 5-(2-methylmercaptoethyl)-hydantoic acid amide,5-(methylmercaptoethyl)-hydantoic acid, methionine amide and in traces,along with other components, 3-methylmercaptopropionaldehydecyanohydrin. These can be converted to methionine in alkalinehydrolysis, just like the main product. An exception is3-methylmercaptopropionaldehyde cyanohydrin, which on hydrolysis isconverted to 4-methylmercapto-2-hydroxybutanoic acid. The precisecomposition of the product mixture produced in the hydantoin reactioncan be elucidated by HPLC.

For complete conversion of MMP to the hydantoin derivative, it isadvantageous if MMP and NH₃ are used in the reactive absorber in a molarratio of about 1 to 3, wherein the basic equation of hydantoin synthesishas the following appearance (formula III):

As can be seen from formula IV, CO₂ as reaction partner is alwayspresent in excess, because during formation of the methionine-potassiumsalt, additionally 0.5 mol CO₂ is released from potassium carbonate.

It is also favourable, for selective reaction of MMP with NH₃, HCN andCO₂ in aqueous phase to hydantoin, if the reaction partners are mixedthoroughly at the moment when they come together. It is thereforepreferable to use a jet washer system as reactive absorber. The amountof circulated process solution is then preferably 18 to 22 times,especially preferably 20 times the total of the amounts of MMP and HCNused. The jet washer ensures very good mass transfer from the gas phaseinto the liquid phase.

To achieve complete conversion of MMP to hydantoin in a reaction timethat is acceptable for industrial use, the hydantoin reaction mixtureleaves the jet washer, and is led through a second reactor, which isdesigned as a flow tube, wherein the residence time in the flow tube isabout 10 minutes. For maximum hydantoin yield, it is important inparticular to avoid formation of the by-product4-methylmercapto-2-hydroxybutanoic acid (=methionine-hydroxy analogue,MHA), which occurs if the residence time is insufficient (formula V):

In the post-reaction phase in the reaction tube, the losses throughformation of 4-methylmercapto-2-hydroxybutanoic acid can be reduced toless than 1 mol. % relative to the MMP used (detection by HPLC).

Alternatively, in hydantoin production it is also possible to usepreviously synthesized MMP-cyanohydrin.

In a subsequent step, the hydantoin derivative is converted in alkalinehydrolysis to methionine. Methionine denotes racemic methionine, whichis also designated as D,L-methionine. This step of the method accordingto the invention is preferably carried out in a sieve plate column,which is operated as reactive distillation.

The reactive distillation column that is operated in the method of theinvention, and is preferably equipped with sieve plates, bringsabout—along with very effective distillation of the ammonia—mainly avery advantageous reaction for alkaline hydrolysis of hydantoin withformation of the potassium salt of methionine. This takes placeaccording to formula IV shown above.

Furthermore, the invention offers the advantage that the amount of NH₃required for operation of the reactive distillation-reactive absorbercoupled system does not have to be prepared and fed in separately, butis circulating in the system. The concentration of ammonia in thebottoms product is preferably less than 120 ppm, more preferably lessthan 100 ppm and most preferably less than 80 ppm. This manner ofoperation means that it is advantageously possible in the “steady state”to dispense with external NH₃ feed completely.

It has been found that when hydantoin hydrolysis is carried out inreactive distillation designed as a sieve plate column, severaladvantageous effects can be achieved.

The inventors' own intensive research into the reaction mechanism ofhydantoin hydrolysis showed, surprisingly, that first there is formationof a stable intermediate, namely the potassium salt of hydantoic acid,according to the following equation (formula VI).

The further hydrolytic degradation of hydantoic acid takes placeaccording to the following equation (formula VII):

As the potassium salt that has formed can react via the equilibriumreaction back to the salt of hydantoic acid, for conversion that is ascomplete as possible, with short reaction times as are requiredindustrially, it is a considerable advantage if ammonia and CO₂ areremoved effectively from the liquid phase during the reaction. A sieveplate column is used for this in one embodiment. For controlling astable NH₃ holdup in the reactive distillation-reactive absorber systemof the invention, in a further embodiment a small partial stream isdeliberately taken from the gaseous top product and discarded. In thisway, the excess NH₃ that forms can be removed from the system in anenvironment-friendly manner and avoiding loss of valuable raw materials.

According to the invention, preferably water, ammonia (NH₃) and CO₂ aredistilled off at the top and then all of the NH₃ or a proportion thereofis condensed and used in hydantoin synthesis, which is preferablydesigned as a reactive absorber, especially preferably as a reactiveabsorber that is operated without losses of NH₃ in the waste gas. Thestream from the bottom of the reactive distillation system contains thealkali salt, preferably the potassium salt of methionine, which isprocessed further to methionine as is known in the background art.

The heating and stripping medium used is preferably steam, which is fedin under pressure below the bottommost sieve plate. The amount, velocityand temperature of the stream of steam are preferably controlled in sucha way that a temperature of 180° C.-190° C. is reached at the outlet ofthe reactive distillation column, whereas the gas phase leaves at thetop of the column at a temperature of 160° C.-170° C. This temperaturerange corresponds to a pressure range of 8-10 bar(excess). Furthermore,the amount of steam depends on the desired throughputs.

The invention further relates to a reaction system for the production ofmethionine salt, which comprises a reactive-rectification column with aweir height of 100 mm or more (also called reactive distillation columnor reactive distillation).

A preferred embodiment has the following design features (see FIG. 1):

Weir height 8: 100-1000 mm, preferably 150-700 mm

Plate spacing 9: 500-1000 mm

Ratio: column diameter 4/weir length 3: 1.1-1.3

Ratio: cross-sectional area/area through which gas flows: 1.5-2

Number of plates: 15-25, especially 18-20.

The cross-sectional area can be calculated from the column diameter 4.The area through which gas flows is found by subtracting the areas ofthe two downcomers 2 from the cross-sectional area.

If the column is a sieve plate column (see FIGS. 1 and 2), the followingfeatures are preferred:

Ratio: total area of all holes/area through which gas flows: 0.04-0.08Diameter of the individual holes in the sieve plate 5-10 mm.

FIG. 1, top, shows a schematic top view of a sieve plate and, bottom, inone section, the side view of a column of a preferred embodiment. Theside view shows the vapour phases 5, liquid/vapour phases 6 and theliquid phases 7 on the individual plates.

The plate spacing can preferably be adjusted to the different reactionphases in the column. In one embodiment, in the top half of the columnthe plate spacing and the weir heights are preferably kept small, tospeed up the stripping of NH₃ and CO₂, whereas in the bottom half theplate spacing and weir heights are larger, for completing the conversionwith a longer residence time.

In a preferred embodiment the weir height for the upper plates of acolumn is in the range from 100 to 200 mm, preferably 150 mm, at a platespacing in the range from 800 to 1000 mm, preferably 1000 mm, and theweir height for the middle plates is in the range from 400 to 600 mm,preferably 500 mm, at a plate spacing in the range from 700 to 900 mm,preferably 800 mm, and the weir height for the bottom plates is in therange from 600 to 800 mm, preferably 700 mm, at a plate spacing in therange from 800 to 1000 mm, preferably 1000 mm. The plate spacing is ineach case measured from one plate to the plate above it. In the case ofcolumns with 15 to 17 plates, preferably plates 1 to 4 count as the topplates, plates 5 to 11 as the middle plates and the lower plates countas the bottom plates. In the case of columns with 18 to 21 plates,preferably plates 1 to 5 count as the top plates, plates 6 to 12 as themiddle plates and the lower plates count as the bottom plates. In thecase of columns with 22 to 25 plates, preferably plates 1 to 7 count asthe top plates, plates 8 to 16 as the middle plates and the lower platesas the bottom plates.

The preferred combination of reactive-rectification column and reactiveabsorber permits, uniquely, the provision of process conditions thatmake the production of methionine quite particularly economicallyattractive.

The reactive-rectification column can have sampling points on one ormore or all plates. Preferably it has sampling points on every fourth,every third, every second to fourth or third, especially preferably onevery second plate. The samples taken are preferably allowed to cool andare analysed for hydantoin, hydantoic acid, methionine,methionyl-methionine by high-performance liquid chromatography (HPLC).The NH₃ content can be determined potentiometrically by means of anion-selective electrode.

It is found in particular that it is possible to operate the preferredembodiment of reactive distillation equipped with sieve plates (seeFIG. 1) with an amount of steam of less than 0.25 t steam per t processsolutions, fed at the top of the column. This is completely unexpected,because at the high pressures in reactive distillation described aboveand the aforementioned large NH₃ circulation from the top of reactivedistillation of about 0.4 t NH₃ per t of MMP employed, much larger NH₃losses via the bottom of the column were to be expected. A lower limitis imposed for the amount of steam used when the so-called weep point ofthe sieve plates is reached. This is understood as the effect thatstarting from a certain reduced gas flow from below, liquid isincreasingly discharged (weeps) directly through the holes of the sieveplates and no longer takes the intended path via the downcomers 2. As aresult, the residence time for the hydrolysis reaction would be lost andthus the desired function of reactive distillation would be disturbed.In detailed studies of the present reaction system it was determinedthat the weep point is equal to 50% of the specific amount of steamstated above. The preferred amounts of steam are in the range from 0.13t steam per t process solution to 0.4 t steam per t process solution,most preferably in the range from 0.20 t steam per t process solution to0.25 t steam per t process solution.

In a preferred embodiment, zirconium is used in the reactivedistillation and/or in the reactive absorber and/or in the secondreactor as material for the parts in contact with the product. As aresult, corrosion damage to the parts in contact with the product can beavoided sustainably. The process combination of reactive distillationand reactive absorption dislcosed here proves particularly advantageous,against the background that zirconium is a high-cost material, becausethe close coupling of the two process steps minimizes the number andlength of connecting pipelines and the number of buffer tanks.Accordingly, the method of the invention is on the whole verysustainable, as it avoids the discharge of environmentally harmful heavymetals such as chromium and nickel due to corrosion. Furthermore, theclose coupling of the process steps means that the waste heat resultingfrom operation of the column can be utilized ideally for warming thefeed streams and additionally for operation of the evaporating unit.

The reaction system according to the invention ensures a sufficientresidence time of the reaction partners in the column. At the same time,in view of the high costs of the material zirconium, it is advantageousif the number of plates can be kept small. The plates are preferablysieve plates. Other usual plate designs (e.g. slotted, valve orbubble-cap plates) can be used, but they have the disadvantage thatmanufacture thereof from the material zirconium is very difficult.Therefore in this application reference is made to sieve plates andsieve plate columns. The invention is not, however, limited to these,but relates equally to slotted, valve or bubble-cap plates or slotted,valve or bubble-cap plate columns, unless expressly stated otherwise.

In one embodiment the reactive distillation column provides quantitativehydrolysis of hydantoin at temperatures between 160° C. and 180° C. inthe pressure range 8 bar(excess) to 10 bar(excess) in a residence timeof less than 10 minutes, wherein the column ensures an average residencetime of less than 0.5 minute per plate. The weir height has an importantinfluence on the residence time of the individual sieve plate (see FIG.1). It was found, surprisingly, that far larger weir heights arepossible in the reaction system of the invention than those described inthe background art (see e.g. Mersmann, Thermische Verfahrenstechnik(Thermal process engineering), p. 222, Springer Verlag, 1980). Weirheights up to max. 60 mm are stated in the latter, whereas weir heightsup to 1000 mm are used in the invention. Therefore in a preferredembodiment the use of zirconium as material is minimized, whilesimultaneously reducing the NH₃ concentration at column outlet to lessthan 100 ppm.

The mother liquor from precipitation of the methionine salt ispreferably used for the alkaline hydrolysis of hydantoin. The motherliquor contains the potassium salts mainly as KHCO₃. This is thenpreferably concentrated, to remove CO₂ and water, which leads to asolution with high potassium carbonate content and therefore increasedbasicity, which is advantageous for the hydrolysis reaction.

The energy required for operation of this evaporation step can beobtained in an especially suitable manner from the waste heat of thereactive distillation-reactive absorption combination. In a preferredembodiment, the amount of water that is required for production of theamounts of steam necessary for operation of the reactive distillation isobtained from the condensate of the evaporation step. In this way thecomplete methionine production process can be operated largely withoutgenerating wastewater, which represents a considerable advantage fromthe environmental standpoint.

According to the method of the invention, for alkaline hydrolysis, thereaction solutions are introduced into the reactive-rectification columnin such a way that only the solution containing5-(2-methylmercaptoethyl)-hydantoin is fed in at the topmost plate andthe alkaline circulating solution is fed on a plate located under that,preferably on the 2nd plate from the top (see FIG. 2). Therefore firstpreferably NH₃, CO₂ and HCN are stripped from the hydantoin solution andrecycled to the hydantoin reaction. As a result, in particular the lossof HCN according to formula II is minimized and at the same timeformation of potassium formate is avoided. Potassium formate, as aneutral salt, is not suitable for supporting the hydrolysis of hydantoinand must therefore be extracted from the potassium circuit. Theassociated losses of potassium must be made up using KOH. The sequentialfeed of hydantoin solution and alkaline circulating solution at the topof the reactive distillation column therefore avoids costs for rawmaterials and waste disposal.

In a preferred embodiment, using a reactive distillation column withsieve plates, advantageously the formation of by-products such asmethionine dipeptide is suppressed. Since formation of methioninedipeptide requires the simultaneous presence of the starting compoundhydantoin and the potassium salt of methionine, efficient separation ofthe reaction partners is advantageous for avoiding this reaction. Thistakes place in an especially suitable manner in a reactive distillationequipped with sieve plates, as remixing is largely prevented in thissystem. However, the invention is not restricted to this, but relatesequally to slotted, valve or bubble-cap plates or slotted, valve orbubble-cap plate columns, unless expressly stated otherwise. In thisway, the proportions present in the hydrolysed reaction mixture are 98mol. % Met and 2 mol. % Met-dipeptide, wherein a residence time of lessthan 10 minutes is required.

The preferred method of the invention of alkaline hydantoin hydrolysisin a sieve plate column minimizes the formation of the by-productMet-dipeptide by exploiting two synergistic effects. On the one hand,the plate design largely prevents remixing and as a result formation ofMet-dipeptide is inhibited, and on the other hand the intensivestripping operation increases the basicity in the reaction solution, asshown by the following reaction equation.

CO₃ ²⁻+H₂O→2OH⁻+CO₂, gas   VIII

An increased proportion of base in its turn accelerates cleavage of theresultant Met-dipeptide. The mechanism is shown in formula IX:

This explains how the multi-stage sieve plate column of the inventionboth minimizes Met-dipeptide formation and supports the hydrolyticdegradation of the dipeptide, which overall greatly reduces the lossesassociated with by-product formation and disposal thereof.

The presence of a sufficient amount of non-volatile basic compounds inthe reaction matrix is important for efficient conversion of hydantointo methionine in the column of the invention. These are potassium salts,such as KOH, potassium carbonate, potassium hydrogen carbonate,potassium-methionine, potassium salt of Met-dipeptide. As thesepotassium salts are very largely recycled, but at the same timepotassium salts of strong acids are also formed in side reactions(formula II and formula V), it is important to monitor the basicity inthe potassium circuit and keep it stable.

In the continuous process of the invention it is therefore important todetermine the basicity, in particular at column outlet. For this, asample is taken and after it has cooled to ambient temperature it issubmitted to classical acid-base titration with an acid as titrant,wherein the titration end point is found between pH 4 and 5. Preferablythe basicity values in the reaction matrix at the end of the reactionare in the range from 2.2-2.8 mol base per kg saponified solution,preferably 2.5 mol base/kg saponified solution. Lower basicities lead toincreased Met-dipeptide formation and higher NH₃ concentrations in thesaponified product. Higher basicity values mean that, relative to theamount of Met produced, increased circulation of alkaline potassiumsalts is required in the process. This is energetically increasinglycostly and therefore conterproductive.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only, and are not intended to belimiting unless otherwise specified.

EXAMPLES Example 1

FIG. 2 provides further explanation of the invention.

The production of hydantoin took place in the reaction system R-1. Theraw materials HCN and MMP were mixed via the feed points S-1 and S-2with about 20-times the amount of hydantoin reaction solution, which wascirculated by pump P-1. Heat exchanger H-1 served for abstraction ofheat and maintains the reactor contents R-1 preferably at a temperatureof 100° C.-120° C. Alternatively, a premix of MMP and HCN or separatelyproduced MMP-cyanohydrin could also be fed into the stream circulated bypump P-1. The resultant combined stream then served as the driving jetin a jet mixer, which brought the partially condensed low-boilingfractions (S-3) from the top of the reactive distillation R-2intensively into contact with the hydantoin process solution andreturned them to reactor R-1. Via stream S-5, which was washed withwater (S-4) until NH₃-free, a CO₂-containing stream left the reactorR-1. This CO₂ stream arose because according to formula IV, in hydantoinhydrolysis in the presence of potassium carbonate, stoichiometrically0.5 mol more CO₂ forms per mol of methionine formed than was requiredfor hydantoin formation according to formula III. The resultant NH₃-freeCO₂-stream was, as described in U.S. Pat. No. 7,655,072 B2, usefullyreturned to the processing section for isolation of crystallinemethionine, as CO₂ was used there for neutralizing the alkaline processsolution.

The liquid hydantoin-containing stream S-6 was led via a second reactorR-3, to complete the conversion. Simultaneously, entry ofMMP-cyanohydrin into the hydrolysis reactor R-2 was prevented, so thatdisturbing formation of 4-methylmercapto-2-hydroxybutanoic acid therewas prevented (see formula V). In order to minimize the energy requiredin the reactive distillation R-2, the hydantoin-containing processstream S-6 was preferably preheated to 130° C. by means of heatexchanger H-2 before entering at the topmost plate of the sieve-platecolumn R-2. On plate 1 of the reactive distillation, first any HCN stillpresent was preferably expelled. This reduced the hydrolysis of the HCNto the potassium salt of formic acid that otherwise occurs in thealkaline in the reactive distillation (formula II).

The hydrolysis of hydantoin to methionine began with the concurrence ofthe hydantoin-containing process solution and stream S-8, which camefrom the concentrated filtrates from processing to the methionine saltsolids S-9. Stream S-8 took as much heat as possible from the bottomsproduct of reactive distillation (heat exchanger H-4) and was thuspreferably heated to 170° C. By means of evaporator unit H-5, steam waspreferably produced for operation of the reactive distillation, usingcondensates from processing (stream S-7) as the source of water.

Via stream S-10, in particular the ammonia balance was regulated in thecoupled system of the invention, because through the use of excess HCNin hydantoin synthesis and hydrolysis thereof to NH₃ and formic acid,additional amounts of NH₃ were formed, which could be withdrawn veryselectively at this point, i.e. without losses of hydantoin ormethionine. Evaporator H-3 produces, from boiler feed water, heatingsteam of pressure stage 3 bar(excess) (130° C.), which was preferablyused in evaporation of the methionine mother liquor. Therefore, ideally,waste heat from the reactive distillation operated at temperaturesbetween 170° C. and 190° C. could be used in further processing, so thatthe complete production process for methionine according to the methodof the invention was energetically extremely favourable and thereforeeconomically attractive.

Example 2

A sieve plate column with a diameter of 1 metre and 18 sieve plates asshown in FIG. 2 was used for continuous production of methionine. Thefollowing table shows the arrangement of the sieve plates, their spacingand weir heights:

Plate No. Weir height [mm] Plate spacing [mm] 1 to 5 150 1000  6 to 12500 800 13 to 18 700 1000

Tank R-1 was operated with a hold-up of 3 m³, and the second reactor hada hold-up of 1 m³. Pump P-1 provided circulation of 42 t/h. At S-1, 442kg/h HCN (16.92 kmol/h) and at S-2, 1688 kg/h MMP (16.24 kmol/h) werefed into this stream. Via S-3, a stream of 6063 kg/h with an NH₃ contentof 11.6 wt. % of condensates from the top of the reactive distillationwas mixed into the circulated process solution. Excess CO₂ gas (400kg/h) left tank R-1 via a washing column, which was fed at the top (S-4)with 770 kg/h water, so that the CO₂ stream was washed until NH₃-freeand could then be recycled.

By means of condenser H-1, the temperature at the outlet of R-1 wascontrolled at 105° C. The hydantoin reaction solution (S-6) was ledthrough the second reactor R-3 for completing the reaction, it washeated in the heat exchanger H-2 to 130° C. and then fed at the topmostplate of the sieve plate column. 14.14 t/h of potassiumcarbonate—containing process solution (S-8), heated by heat exchangerH-4 to 170° C., containing the following components: 66 g/kg Met, 158g/kg potassium, 48 g/kg Met-Met, 6.5 g/kg MHA, 12.5 g/kg formate, 3.6mol base/kg for basicity, was pumped onto plate 2.

The stream of steam required for operation (5470 kg/h) was produced byevaporator H-5 (S-7) and was fed in below the bottommost sieve plate.From the stream distilled at the top, a partial stream S-10 (54 kg/h)was diverted and discarded.

The pressure at the top of the sieve plate column was 8.2 bar(excess)and the temperature at the inlet of the heat exchanger H-3 was 165° C.The differential pressure over all sieve plates was 450 mbar.

The temperature in the bottom of the column was 189° C.

At column outlet there was a stream S-9 of 22.15 t/h, which contains thefollowing components: 149 g/kg Met, 32 g/kg Met-Met, 101 g/kg potassium,8.2 g/kg formate, 4.2 g/kg MHA and with a basicity of 2.5 mol base/kg.

The sieve plate column was equipped with sampling points on plates 2, 4,6, 8, 10, 12, 14, 16 and 18. The samples were cooled immediately andwere analysed by high-performance liquid chromatography (HPLC) forhydantoin, hydantoic acid, methionine, methionyl-methionine. The NH₃content was determined potentiometrically by means of an ion-selectiveelectrode. The composition on the various sieve plates is presented inthe following table and in FIG. 3.

Sieve Hydantoic plate Methionine NH₃ Hydantoin acid Methionyl-Met No.[g/kg] [g/kg] [g/kg] [g/kg] [g/kg] 2 31 14.2 131 18 30 4 69 8.4 91 23 326 101 5 56 26 34 8 122 2.4 34 19 35 10 141 1.5 15.1 10 35.5 12 158 0.76.4 5 36 14 164 0.6 3.2 2.4 35 16 166 0.2 1.2 0.6 33 18 167 0.1 0 0 32

For determining the residence time, the liquids content was determinedcontinuously, by first stopping streams S-6, S-7, S-8 and S-9simultaneously. After waiting until no further increase in level wasrecorded in the column bottom, the initial level was restored in thecolumn bottom by pumping out. The amount of solution pumped out was 2.4t. Since the throughput in continuous operation was 22.15 t/h, this gavea residence time of the liquid phase of 6.5 min.

U.S. provisional application 61/529,013 filed Aug. 30, 2011, areincorporated herein by reference.

Numerous modifications and variations on the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

1. A reaction system, comprising: a reactive rectification columncomprising a weir having a height of 100 mm or more; wherein saidreaction system is suitable for production of a methionine salt.
 2. Thereaction system according to claim 1, wherein said reactiverectification column further comprises: a plate; wherein the weir heightis in the range from 100 to 1000 mm a plate spacing is in the range from500 to 1000 mm, the ratio of column diameter to weir length is in therange from 1.1 to 1.3, a ratio of a cross-sectional area of the columnto an area through which a gas flows is in the range from 1.5 to 2, andthe number of plates is in the range from 15 to
 25. 3. The reactionsystem according to claim 1, wherein said reactive rectification columnis a sieve plate column, perforated plate column, valve plate column orbubble plate column.
 4. The reaction system according to claim 1,wherein said reactive rectification column further comprises: at leastone plate; wherein the ratio of a total area of all holes of said atleast one plate/area through which a gas flows is in the range from 0.04to 0.08, and a diameter of the individual holes in the plate is in therange from 5 to 10 mm.
 5. The reaction system according to claim 1,wherein the weir height of said reactive rectification column is suchthat an average residence time of a reaction mixture is less than 0.5min per plate.
 6. The reaction system according to claim 1, furthercomprising: at least one reactive absorber; and optionally, a secondreactor for the production of 5-(2-methylmercaptoethyl)-hydantoin. 7.The reaction system according to claim 6, wherein the reactive absorberis a jet washer system.
 8. The reaction system according to claim 6,wherein zirconium is used in the reactive-rectification column and/or inthe reactive absorber and/or in the second reactor as material for apart in contact with a product.
 9. The reaction system according toclaim 1, wherein said reactive rectification column further comprises:an upper plate, a middle plate and a bottom plate; wherein the weirheight for the upper plate is in the range from 100 to 200 mm at a platespacing in the range from 800 to 1000 mm, the weir height for the middleplate is in the range from 400 to 600 mm at a plate spacing in the rangefrom 700 to 900 mm, and the weir height for the bottom plate is in therange from 600 to 800 mm at a plate spacing in the range from 800 to1000 mm.
 10. The reaction system according to claim 2, wherein the weirheight is in the range from 150 to 700 mm, and the number of plates isin the range from 18 to
 20. 11. The reaction system according to claim8, wherein zirconium is used as material for all parts in contact withthe product.
 12. A method for continuous production of a methioninesalt, comprising: reacting 3-methylmercaptopropionaldehyde and hydrogencyanide or a component that can be produced therefrom, thereby obtaininga solution containing 5-(2-methylmercaptoethyl)-hydantoin; alkalinehydrolysing the 5-(2-methylmercaptoethyl)-hydantoin to a methionine saltin a reactive rectification column, wherein only the solution containing5-(2-methylmercaptoethyl)-hydantoin is fed on a topmost plate of thereactive rectification column and an alkaline circulating solution isfed on a plate located under the topmost plate.
 13. The method accordingto claim 12, wherein the alkaline circulating solution contains analkali metal carbonate.
 14. The method according to claim 12, whereinwater, ammonia and CO₂ are removed from the top of the reactiverectification column and the NH₃ removed is condensed completely orpartially and used in the synthesis of5-(2-methylmercaptoethyl)-hydantoin.
 15. The method according to claim12, wherein a concentration of ammonia at the bottom of therectification column is less than 120 ppm.
 16. The method according toclaim 12, wherein the reaction to 5-(2-methylmercaptoethyl)-hydantoin iscarried out in a reactive absorber and then in a second reactor.
 17. Themethod according to claim 12, wherein the reaction to5-(2-methylmercaptoethyl)-hydantoin is carried out in a reactiveabsorber and then in a second reactor designed as a flow tube.
 18. Themethod according to claim 12, wherein the temperature of the reactionmixture at the outlet of the reactive-rectification column is in therange from 180° C. to 190° C.
 19. The method according to claim 12,wherein the temperature of the gas phase at the top of thereactive-rectification column is in the range from 160° C. to 170° C.20. The method according to claim 12, wherein alkaline hydrolysis iscarried out at a pressure in the range from 8 bar(excess) to 10bar(excess).
 21. The method according to claim 12, wherein steam is usedas heating and stripping medium in the reactive-rectification column.22. The method according to claim 12, which is carried out in a reactionsystem according to claim
 1. 23. The method according to claim 12,wherein said alkaline circulating solution is fed on the 2nd plate fromthe top.
 24. The method according to claim 12, wherein the alkalinecirculating solution contains potassium carbonate.
 25. The methodaccording to claim 12, wherein allyl alcohol is removed at the top ofthe reactive rectification column.